In a typical optical amplifying device, when part of amplified signal light is reflected along an optical path on the output side, this reflected return light may cause resonant behavior through an optical path loop that passes through an optical amplifier. As a technique that suppresses the resonant behavior occurring due to the reflected return light, a structure is known in which an optical isolator is disposed in the vicinity of the input or output end of an optical amplifying device.
Here, a brief description will be given of the resonant behavior of an optical amplifying device and the function of an optical isolator therein, with reference to FIGS. 1A to 1C. When there are two reflection points R1 and R2 on an optical path along which a signal light Ls travels in one direction as illustrated in FIG. 1A, part of the signal light Ls is reflected at the downstream reflection point R2, and reflected return light Lr, which is the reflected part of the signal light Ls, propagates along the optical path in a direction opposite to that in which the signal light Ls travels. This reflected return light Lr is reflected at the upstream reflection point R1 again, and then propagates along the optical path in a direction the same as that in which the signal light Ls travels. Further, the component of the reflected return light Lr which has passed through the reflection point R2 becomes crosstalk light Lxt for the original signal light Ls.
In the case where an optical amplifying device (AMP) is disposed on the optical path between the reflection points R1 and R2 as illustrated in FIG. 1B, the signal light Ls is amplified by the optical amplifying device, and in turn, the reflected return light Lr is amplified twice at each reciprocation. Note that in FIGS. 1A to 1C, the thickness of each arrow represents a light intensity thereof. When the reflection points R1 and R2 are present on the respective sides of the optical amplifying device as described above, the intensity of the crosstalk light Lxt may be increased by the resonant structure created between the reflection points. As a result, there are cases where the quality of the signal light Ls output from the optical amplifying device is deteriorated.
In order to decrease the intensity of the crosstalk light Lxt which has been increased in this manner, it is effective to dispose an optical isolator (ISO) on the optical path between the reflection points R1 and R2 as illustrated in FIG. 1C. For example, in FIG. 1C, the optical isolator is disposed on the signal output side of the optical amplifying device. This optical isolator has a property of allowing light propagating in the same direction as that in which the signal light Ls propagates to pass therethrough, but blocking light propagating in the opposite direction. In this configuration, the signal light Ls is delivered to the optical isolator after being amplified by the optical amplifying device, and then, part of the signal light Ls having passed through the optical isolator is reflected at the reflection point R2.
The reflected return light Lr is delivered to the optical isolator in the direction opposite to that in which the signal light Ls travels. Accordingly, almost all components of the reflected return light Lr are blocked by the optical isolator. Thus, the reflected return light Lr that would reach the reflection point R1 through the optical amplifying device is substantially extinguished. This results in effectively decreasing the intensity of the crosstalk light Lxt, which may have been increased in the above manner. Although FIG. 1C illustrates the example in which the optical isolator is disposed on the signal output side of the optical amplifying device, it is possible to produce the same effect by disposing the optical isolator on the signal input side thereof.
As an example of the related art regarding the application of an optical isolator to an optical amplifying device as described above, for example, Japanese Laid-open Patent Publication No. 2005-19639 discloses a structure of an optical module that generates laser light, in which a first optical isolator and a second optical isolator are arranged on an optical axis of laser light output from a semiconductor laser diode. The first optical isolator in the optical module has a light shielding property, the center wavelength of which is designed to block light of the same wavelength as the light emitting wavelength of the semiconductor laser diode.
Meanwhile, the second optical isolator has a light shielding property, the center wavelength of which is designed to block light of a wavelength shorter than the light emitting wavelength of the semiconductor laser diode. This structure enables the first optical isolator to block the reflected return light of the laser light, such as light of a 1.55 μm band, which the optical module outputs to an optical fiber, and in turn, the second optical isolator to block external noise light of a wavelength shorter than the light emitting wavelength of the semiconductor laser diode, such as excitation light of a 1.48 μm band. This makes it possible to suppress the influences of the reflected return light of the laser light and the external noise light upon the semiconductor laser diode, for example, upon the destabilization of the internal laser oscillation in the semiconductor laser diode.
A typical optical isolator has an isolation of about −30 dB for light traveling in the direction in which light is blocked, and this isolation depends on the wavelength of incoming light. Note that the isolation I [dB] of an optical isolator is defined by,I=10·log(Pout/Pin),where Pin [W] is the power of input light traveling in the blocking direction, and Pout [W] is the power of output light traveling in the blocking direction.
FIG. 2 depicts an example of an isolation-wavelength property of a typical optical isolator by using a solid line. As depicted in FIG. 2, a typical isolation-wavelength property I has a minimum peak of the isolation at a given wavelength. Accordingly, a wavelength bandwidth of light (thereinafter, called an “isolation bandwidth”) which an optical isolator may substantially block is limited to a given wavelength range having, at a center thereof, a wavelength peak at which the isolation is a minimum. For example, when an optical isolator having an isolation performance where the light transmittance is equal to or less than −30 dB is requested, a wavelength range IB depicted in FIG. 2 becomes an isolation bandwidth.
When an optical amplifying device has a gain bandwidth narrower than the isolation bandwidth of the above optical isolator, a single isolator may be used to decrease the intensity of the crosstalk light as illustrated in FIG. 1C. Alternatively, a plurality of optical isolators having the same optical property may be used in combination. Meanwhile, when an optical amplifying device has a gain-wavelength property G as indicated by a dotted line in FIG. 2, and a gain bandwidth GB thereof is wider than the isolation bandwidth IB of the optical isolator, the components of reflected return light which fall within the gain bandwidth GB but outside the isolation bandwidth IB are not blocked sufficiently. In this case, it is disadvantageously difficult to decrease the intensity of the crosstalk light by using a single optical isolator. This disadvantage becomes significant in an optical amplifying device that is known to have a wide gain bandwidth, like one including a combination of a semiconductor optical amplifier (SOA) and a plurality of optical amplifiers having different amplifying bandwidths.
As for the above disadvantage, considering the case where the structure of the above optical module generating laser light with the first and second optical isolators is applied to an optical amplifying device having a wide gain bandwidth, the first optical isolator may block part of light, the wavelength of which falls within the gain bandwidth, because the light emitting wavelength of the semiconductor laser diode in the optical module corresponds to the gain bandwidth of the optical amplifying device.
However, since the second optical isolator is configured to block light of wavelengths falling outside the gain bandwidth, it is difficult to decrease the intensity of the remaining light, or the crosstalk light, which has a wavelength falling within the gain bandwidth and has been not blocked by the first optical isolator. Thus, the related art regarding the suppression of resonant behavior by using an optical isolator has failed to efficiently suppress the resonant behavior occurring due to the reflected return light of the signal light, when an optical amplifying device that may amplify signal light of a wide wavelength bandwidth as a whole is used.